Pentafluoroethane is one of the possible replacements for chlorofluorocarbons (CFCs) to which the Montreal Protocol applies and which are characterized by an exceptionally long lifetime, enabling them to reach the high layers of the atmosphere and thus to contribute, under the influence of the UV radiation, to the destruction of the ozone layer. It is therefore obvious that their replacements must contain only traces of these CFCs, depending on the various processes for obtaining them.
The replacements are generally obtained either by appropriate fluorination methods, which are not highly selective and can give rise to perhalogenated compounds of the CFC type by disproportionation, or from CFCs themselves by reduction methods, in practice by hydrogenolysis reactions. Pentafluoroethane (F125) can thus be prepared by fluorination of perchloroethylene or of its intermediate fluorination products such as dichlorotrifluoroethane (F123) and chlorotetrafluoroethane (F124) or by hydrogenolysis of chloropentafluoroethane (F115). In both cases the F125 produced contains significant quantities of F115 which must be removed as completely as possible, since F115 is a CFC.
However, the existence of an F115/F125 azeotrope containing 21 % by weight of F115 (see U.S. Pat. No. 3,505,233) with a boiling point (-48.5.degree. C. at 1.013 bar) very close to that of F125 (-48.1.degree. C.) makes complete separation of F115 and of F125 by distillation practically impossible unless technically complex procedures are employed, such as azeotropic distillation at various pressures, as described in U.S. Pat. No. 5,346,595. The removal of F115 from F125 can therefore be done only by a chemical route or by physical methods involving a third substance.
In patent application EP 508 631, which describes the production of hydrofluorocarbons (HFCs) by liquid-phase chemical reduction of chlorine, bromine or iodine compounds with a metal hydride or a complex of such a hydride, it is indicated that this process can be advantageous for purifying some HFCs like F125. For the same purpose, Japanese patent application (Kokai) published under No. 2001414/90 employs metal redox couples in a solvent medium. Other techniques, like that described in Journal of Fluorine Chemistry, 1991 vol. 55, p. 105-107, employ organic reducing agents such as ammonium formate in DMF medium and in the presence of ammonium persulphate.
These processes, which make use of reactants that are difficult to handle (metal hydrides) or are liable to present effluent problems, are not very compatible with industrial production of F125 in large tonnages.
For industrial manufacture of F125 the extractive distillation technique appears to be the ideal process for removing the residual F115.
In an extractive distillation process the separation of the constituents of a binary mixture is done with the aid of a so-called extraction column comprising successively, from the boiler to the top, three sections, one for exhaustion, the second for absorption and the third for recovery.
The binary mixture to be fractionated is injected at the top of the exhaustion section while the third substance acting as a selective solvent is introduced at the top of the absorption section so as to travel in the liquid state from its point of introduction to the boiler.
The third, so-called recovery section is used to separate by distillation the least absorbed constituent, from the traces of solvent which are entrained under the effect of its vapour pressure, which is not zero.
The application of this technique to the purification of 1,1,1,2-tetrafluoroethane (F134a) has formed the subject-matter of U.S. Pat. No. 5,200,431; the extractant employed is a chlorinated solvent or an aliphatic hydrocarbon.
The application of extractive distillation to the purification of F125 is already described in a number of documents including U.S. Pat. No. 5,087,329, which employs as extractant a C.sub.1 -C.sub.4 fluorohydrocarbon optionally containing hydrogen and/or chlorine atoms and which has a boiling point of between -39 and +50.degree. C. According to the data in this patent, dichlorotetrafluoroethanes (F114 and F114a) are at least three times more efficient than the other compounds cited. Furthermore, 5 of the 8 solvents cited are CFCs to which the Montreal Protocol applies and whose commercial availability should cease in the near future.
Industrial use of the process according to the patent can therefore be envisaged economically only when the extractant used forms part of the chain of inter-mediates leading to F125, that is to say actually in the processes for the preparation of F125 by hydrogenolysis.
In the case of manufacture of F125 by fluorination of perchloroethylene or of its partial fluorination products (F122, F123, F124), U.S. Pat. No. 5,087,329 leaves a choice only between CFCs, which will no longer be found on the market, and less efficient products such as F124 or F123.
Other purification processes making use of the same technique have been described in patent applications EP 626 362, WO 9521147, WO 9521148, FR 2 716 449, WO 9606063 and WO 9607627; the extractants employed are hydrocarbons or polar solvents (alcohols, ethers, ketones or esters). In the case where polar solvents are used the selectivity is reversed, that is to say that the compound entrained by the solvent is no longer the impurity (F115) but the product to be purified (F125); this procedure is obviously more clumsy from an industrial viewpoint. Furthermore, all these extractants have the disadvantage of being highly flammable and therefore require industrial plants that are much more costly from the viewpoint of safety.